The invention relates to an induction melting furnace sealed against the atmosphere and having a connection for attachment to a vacuum source or inert gas source, for melting reactive metals of high melting point, which has an unlined melting crucible of metal within an induction coil, and a casting mold disposed beneath the crucible under an outlet.
Such an induction melting apparatus is described, for example, in U.S. Pat. No. 5,121,406. In it the crucible and the mold are disposed one above the other in a relatively large chamber so that air can be excluded from the molten metal during the melting and during the filling of the mold.
The crucible is in the form of a so-called "cold induction crucible" and has a wall formed of individual palisades. The palisades consist usually of copper and are water-cooled. A typical embodiment of such a "cold induction crucible" is described in U.S. Pat. No. 4,738,713. In this patent it is also stated that, for the performance of the remelting and casting the induction crucible and the mold must be contained in an air-tight chamber.
The known induction melting apparatus are relatively bulky and expensive to manufacture due to the chamber necessary for keeping the air out. Also, to produce a vacuum a comparatively large amount of air has to be aspirated out of the chamber, so that correspondingly large vacuum pumps are necessary and a considerable amount of time is needed after the closing of the chamber before the necessary vacuum has been produced.
The invention is addressed to the problem of improving an induction melting apparatus of the kind described above so that it will be as compact as possible, will be less expensive to manufacture, and require very brief intervals of auxiliary process time.
This problem is solved according to the invention in that the crucible has a cover that closes it hermetically and a tubular downward prolongation in which the mold is disposed so as to be sealed to the interior surface of the prolongation.
With this configuration it is possible to do away entirely with the chamber surrounding the crucible and the mold. It becomes possible, due to the cover and the sealed arrangement of the mold in the prolongation of the crucible to evacuate or to inject an inert gas into the interior of the crucible and mold, in which case the crucible forms the vacuum chamber. Thus the induction melting apparatus is of much more compact construction than comparable apparatus. Furthermore, it can be made ready for operation more quickly. Also, the individual parts of the apparatus are more accessible due to the absence of this enveloping chamber, and this is advantageous in the handling and maintenance of the apparatus.
If it is desired to remelt smaller amounts of metal in a crucible, it is possible by means of the magnetic forces produced by the induction coil to bring the molten metal into a suspended state out of contact with the wall of the crucible. This advantageous "float fusion" can still be achieved even in the case of amounts of several hundred grams of metal if, according to an advantageous embodiment of the invention, the crucible has a funnel-shaped bottom tapering toward its outlet. By means of such a bottom upwardly directed components of the magnetic forces produced by the induction coil can be achieved, resulting in the levitation of the melt.
In larger melting furnaces which cannot operate on the float fusion principle, after the crucible cools off the problem exists of removing the skull from the crucible. It can very easily be pulled out of it if the crucible has a perimeter that increases slightly in diameter toward the top.
One very especially advantageous embodiment of the invention consists in making the mold so as to be able to be raised and lowered in the prolongation. This makes it possible to hold the mold ready in a lower part of the prolongation before pouring, where it is heated increasingly from its bottom up. When the mold has been lifted this results in a directed solidification, which considerably improves the quality of the casting.
The movement of the mold in the prolongation can be achieved in an especially simple manner if it is disposed on a ram extending into the prolongation.
In the case of larger amounts of metal it is no longer possible for the melt to be held by magnetic forces and its surface tension above the outlet of the crucible. It can be kept from flowing out until the desired time for the mold to be filled simply by providing at the outlet of the crucible a plug which can be melted out by an additional heater or by shutting off a cooling system provided for it.
It is also possible, however, in the case of larger amounts of metal, to prevent premature outflow of the melt by providing the mold with a connection for connecting it to a vacuum or an inert gas.